1. Field of the Invention
This invention relates to radio frequency communication and more particularly to low noise amplifiers used in radio frequency signal transmission systems.
2. Description of Related Art
Low noise amplifiers (LNAs) are used in conjunction with numerous radio frequency functions, for example, mixers, voltage controlled oscillators, limiters, filters, etc. As such, the cost of a LNA is significant to the overall cost of the function. Generally, the cost of a LNA is determined by two factors: (1) the integrated circuit (IC) cost; and (2) the cost of off-chip components that are necessary to the operation of the LNA and to the matching of the LNA with other radio frequency functions. Currently, to achieve a low noise amplifier that is easily integrated with other radio frequency functions, it has been necessary to use a multitude of off-chip components which inherently increases the expense. Alternatively, application specific integrated circuits (ASICs) can be fabricated to include all the necessary components onto one chip, however, this process is an expensive, detailed fabrication process.
Further, while state-of-the-art LNAs are capable of providing a noise figure approaching 1.5 dB they generally use 45 to 90 mW of power, e.g. see Mini-Circuits Corporation model VAM-6, an undesirably high figure.
In an attempt to address some of the concerns described above, U.S. Pat. No. 5,574,405 describes a low noise amplifier (LNA)/mixer/frequency synthesizer circuit for an RF system. The LNA/mixer/frequency synthesizer circuit of the '405 patent is fabricated as an integrated circuit using CMOS technologies. In the '405 patent, the LNA portion of the circuit comprises a first transistor directly coupled to an antenna port, a current mirror to set a predetermined bias current to the first transistor and an inductor, which couples the first transistor to the frequency synthesizer circuit. The '405 patent does not address integrating the LNA with radio functions other than the mixer and frequency synthesizer circuits and, as such, does not provide for both input and output impedance matching. Further, the LNA provides only a single stage of amplification, limiting the overall range of programmable amplification.
As such, there is a need for a LNA that can reduce the number of off-chip components and utilize an IC fabricated with low-cost, CMOS technology to significantly reduce the cost of the LNA and the overall cost of the radio frequency functions with which the LNA is integrated while at the same time provide a noise figure comparable to current, state-of-the-art LNAs, reduced power consumption and a broad range of programmable amplification.